Technique for forming the deep doped columns in superjunction

ABSTRACT

A method of manufacturing a semiconductor device is disclosed and starts with a semiconductor substrate having a heavily doped N region at the bottom main surface and having a lightly doped N region at the top main surface. There are a plurality of trenches in the substrate, with each trench having a first extending portion extending from the top main surface towards the heavily doped region. Each trench has two sidewall surfaces in parallel alignment with each other. A blocking layer is formed on the sidewalls and the bottom of each trench. Then a P type dopant is obliquely implanted into the sidewall surfaces to form P type doped regions. The blocking layer is then removed. The bottom of the trenches is then etched to remove any implanted P type dopants. The implants are diffused and the trenches are filled.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 11/343,329, filed Jan. 31, 2006, currently pending, entitled “ATechnique for Forming the Deep Doped Columns in Superjunction,” which isa continuation of U.S. patent application Ser. No. 10/857,323, filed May28, 2004, now U.S. Pat. No. 7,015,104, entitled “A Technique for Formingthe Deep Doped Columns in Superjunction,” which claims the benefit ofU.S. Provisional Patent Application No. 60/474,127, filed on May 29,2003, entitled “A Technique for Forming the Deep Doped Columns inSuperjunction.”

BACKGROUND OF THE INVENTION

The present invention relates generally to semiconductor devices, andmore, particularly, to power MOSFET devices.

Power MOSFET devices are employed in applications such as automobileelectrical systems, power supplies, and power management applications.Such devices should sustain high voltage in the off-state while having alow voltage drop and high current flow in the on-state.

FIG. 1 illustrates a typical structure for an N-channel power MOSFET. AnNepitaxial silicon layer 1 formed over an N⁺ silicon substrate 2contains P-body regions 5 a and 6 a, and N⁺ source regions 7 and 8 fortwo MOSFET cells in the device. P-body regions 5 and 6 may also includedeep P-body regions 5 b and 6 b. A source-body electrode 12 extendsacross certain surface portions of epitaxial layer 1 to contact thesource and body regions. The N-type drain for both cells is formed bythe portion of N-epitaxial layer 1 extending to the upper semiconductorsurface in FIG. 1. A drain electrode is provided at the bottom of N⁺substrate 2. An insulated gate electrode 18 typically of polysiliconlies primarily over the body and portions of the drain of the device,separated from the body and drain by a thin layer of dielectric, oftensilicon dioxide. A channel is formed between the source and drain at thesurface of the body region when the appropriate positive voltage isapplied to the gate with respect to the source and body electrode.

The on-resistance of the conventional MOSFET shown in FIG. 1 isdetermined largely by the drift zone resistance in epitaxial layer 1.The drift zone resistance is in turn determined by the doping and thelayer thickness of epitaxial layer 1. However, to increase the breakdownvoltage of the device, the doping concentration of epitaxial layer 1must be reduced while the layer thickness is increased. Curve 20 in FIG.2 shows the on-resistance per unit area as a function of the breakdownvoltage for a conventional MOSFET. Unfortunately, as curve 20 shows, theon-resistance of the device increases rapidly as its breakdown voltageincreases. This rapid increase in resistance presents a problem when theMOSFET is to be operated at higher voltages, particularly at voltagesgreater than a few hundred volts.

FIG. 3 shows a MOSFET that is designed to operate at higher voltageswith a reduced on-resistance. This MOSFET is disclosed in paper No. 26.2in the Proceedings of the IEDM, 1998, p. 683. This MOSFET is similar tothe conventional MOSFET shown in FIG. 2 except that it includes P-typedoped regions 40 and 42 which extend from beneath the body regions 5 and6 into the drift region of the device. The P-type doped regions 40 and42 define columns in the drift region that are separated by N-type dopedcolumns, which are defined by the portions of the epitaxial layer 1adjacent the P-doped regions 40 and 42. The alternating columns ofopposite doping type cause the reverse voltage to be built up not onlyin the vertical direction, as in a conventional MOSFET, but in thehorizontal direction as well. As a result, this device can achieve thesame reverse voltage as in the conventional device with a reduced layerthickness of epitaxial layer 1 and with increased doping concentrationin the drift zone. Curve 25 in FIG. 2 shows the on-resistance per unitarea as a function of the breakdown voltage of the MOSFET shown in FIG.3. Clearly, at higher operating voltages, the on-resistance of thisdevice is substantially reduced relative to the device shown in FIG. 1,essentially increasing linearly with the breakdown voltage.

The improved operating characteristics of the device shown in FIG. 3 arebased on charge compensation in the drift region of the transistor. Thatis, the doping in the drift region is substantially increased, e.g., byan order of magnitude or more, and the additional charge iscounterbalanced by the addition of columns of opposite doping type. Theblocking voltage of the transistor thus remains unaltered. The chargecompensating columns do not contribute to the current conduction whenthe device is in its on-state. These desirable properties of thetransistor depend critically on the degree of charge compensation thatis achieved between adjacent columns of opposite doping type.Unfortunately, non-uniformities in the dopant gradient of the columnscan be difficult to avoid as a result of limitations in the control ofprocess parameters during their fabrication. For example, diffusionacross the interface between the columns and the substrate and theinterface between the columns and the P-body region will give rise tochanges in the dopant concentration of the portions of the columns nearthose interfaces.

The structure shown in FIG. 3 can be fabricated with a process sequencethat includes multiple epitaxial deposition steps, each followed by theintroduction of the appropriate dopant. Unfortunately, epitaxialdeposition steps are expensive to perform and thus this structure isexpensive to manufacture. Another technique for fabricating thesedevices is shown in co-pending U.S. application Ser. No. 09/970,972, inwhich a trench is successively etched to different depths. A dopantmaterial is implanted and diffused through the bottom of the trenchafter each etching step to form a series of doped regions (so-called“floating islands”) that collectively function like the P-type dopedregions 40 and 42 seen in FIG. 3. However, the on-resistance of a devicethat uses the floating island technique is not as low as an identicaldevice that uses continuous columns.

Accordingly, it would be desirable to provide a method of fabricatingthe MOSFET structure shown in FIG. 3 that requires a minimum number ofdeposition steps so that it can be produced less expensively while alsoallowing sufficient control of process parameters so that lightly dopedcolumns that extend almost through a layer of deposited can be formed.

SUMMARY OF THE INVENTION

A method of manufacturing a semiconductor device is disclosed and startswith a semiconductor substrate having a heavily doped N region at thebottom main surface and a lightly doped N region at the top mainsurface. There are a plurality of trenches in the substrate, with eachtrench having a first extending portion extending from the top mainsurface towards the heavily doped region. Each trench has two sidewallsurfaces in parallel alignment with each other. A blocking layer isformed on the sidewalls and the bottom of each trench. Then a P typedopant is obliquely implanted into the sidewall surfaces to form P typedoped regions. The blocking layer is then removed. The bottom of thetrenches is then etched to remove any implanted P type dopants. Theimplants are diffused and the trenches are filled.

The body and the source regions are then formed after the gatedielectric and the gate conductor are formed. The body region consistsof an implanted P body region on top of each of the diffused P typedoped regions to form the body regions and implanted N⁺ regions withinthe body regions to form source regions. Above the gate dielectricregion is a gate conductor that extends over the P-type body and the N⁺source regions of two adjoining trenches. A source conductor isconnected to the P-type body and the N⁺ source region.

The un-doped sidewalls will typically be doped with N type dopant. Thetrenches may have the shape of a dog bone, a rectangle, a rectangle withrounded ends or a cross with the P type dopant being implanted into theends of the dog bone, a rectangle, or a rectangle with rounded ends, andin opposite sides of the cross.

Rectangular-shaped trenches may be arranged in an array of rows andcolumns with the ends of the trenches in the column being implanted withP type dopants and the ends of the trenches in the rows being implantedwith N type dopants. Cross-shaped trenches may be implanted with P-typedopant along one set of axes, and with N-type dopant along a second setof axes at 90° to the first set.

The angle of the implant can be selected so that the bottoms of thetrenches are not implanted.

The technique may be used to manufacture the termination regions byvarying the shape, the depth and width of the trenches, in conjunctionwith the implant angle.

The identification of the type of doping use herein only refers to thatshown in the particular embodiment. Those skilled in the art know thatsimilar results may be achieved by using P type dopant instead of N typeand visa versa. The use of the particular type of dopant in thedescription of the embodiments should in no way limit the scope of theclaims.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a sectional view of a prior art conventional MOSFET;

FIG. 2 is a chart showing breakdown voltage, the on-resistances andcurrent;

FIG. 3 is a sectional view of a prior art superjunction transistor;

FIGS. 4-14 illustrate the process steps used to manufacture thedisclosed semiconductor device;

FIGS. 15, 23, 24 and 25 illustrate the different shapes that can be usedto manufacture the disclosed semiconductor device;

FIGS. 16, 18, and 19 illustrate the different arrangements of thetrenches to achieve the disclosed device;

FIG. 17 is a sectional view of the disclosed device illustrating thesource region;

FIGS. 20, 26 and 27 illustrate possible termination arrangements of thesemiconductor device;

FIG. 21 shows a top view of the termination region; and

FIG. 22 is a sectional view of FIG. 21.

DETAILED DESCRIPTION OF THE INVENTION

A technique for forming lightly doped columns that extend almost througha layer of deposited epitaxial semiconductor material is best understoodby referring to FIGS. 4-18 while reading the description below. Thistechnique uses trenches etched into the silicon to form lightly dopedcolumns. One type of trenches has a dimension in a first direction thatis greater than the dimension in a second direction that isperpendicular to the first direction and is generally rectangularshaped, while a second type is cross-shaped. FIG. 16 shows the top viewof a series of generally rectangular shaped trenches 35 following twoseparate implantation steps that have doped the two narrow walls 31 and33 of the trenches 35. FIGS. 9 and 10 show the technique that is used toperform the two implantation steps. The two separate implantation stepsare performed at an angle with respect to the surface of the substratethat allows the dopant to be implanted into just the two narrow “end”sidewalls 31 and 33. The presence of a layer of material such as silicondioxide or silicon nitride (or a sandwich of such materials) preventsthe ions that are being implanted from reaching the semiconductorsidewalls 37 that are along the long axis of each trench. Following theimplantation step, any dopant that has been implanted in the bottom ofthe trench may be removed by etching the trench deeper, and then thedopant may be diffused until the desired dopant distribution isobtained. The trench is then filled using an oxidation or depositionstep.

The shape of the trench is not limited to just being rectangular. Manyother possible trench shapes such as dog-bones 235, or rectangles withrounded ends 135 (FIG. 15), or crosses are also possible. The profile ofthe implanted dopant is slightly different, allowing the optimization ofthe shape of the implanted region. Both of the trench geometries avoidplacing dopant atoms near a corner, which might result in better controlof the resulting dopant profile.

The pattern of trenches across the surface of the device may also bevaried to obtain the best performance. Examples of trench placement areshown in FIG. 16 which shows a square array, FIG. 18 which shows astaggered array 110 and FIG. 19 which illustrates an array 133 of rowsand columns. The number and locations of the trenches is importantbecause it affects overall device efficiency.

One fabrication sequence for the doped columns will now be discussed.

Referring to FIG. 4 a lightly doped epitaxial layer 1 is deposited on aheavily doped substrate 2. Then as shown in FIG. 5 a blocking layer 41of silicon dioxide is either grown or deposited on the top surface ofthe epitaxial. The blocking layer has a desired thickness of between 400and 2,000 A°. In FIG. 6 the blocking layer 41 is masked by a mask 43 tofacilitate its etching. Following the etching of the blocking layer 41,trenches 45 are etched into the epitaxial layer 1 as illustrated in FIG.7. A blocking layer 47 is grown or deposited on all of the sidewalls andbottoms of each trench 45 as is shown in FIG. 8. The thickness ofblocking layer 47 is between 200 and 2000 A°.

Referring to FIG. 9, a first implant of boron ions is performed in thenarrow end 33 at an angle alpha that in conjunction with the thicknessof the blocking layer 47 will limit the penetration of the dopant in tothe epitaxial 1. The thickness of the blocking layer 41 is sufficientenough to prevent the penetration of the dopant into the tops of thecolumns 21. The result is implanted ions 51 in the column 21 at thesmall side 33. Generally to prevent the penetrations of the ions in thebottom of the trench alpha should be equal to the tangent G, the depthof the trench T, the width of the trench.

In FIG. 10 a second implant using the same dopant species is performedat the other small side 31 of the trenches 45 at an angle beta that istraditional equal to alpha minus 90 degrees leaving implanted ions 52 inthe small side 31 as is shown in FIGS. 10 and 11.

The implants are performed parallel to the long axis, the F side, of thegeometry that is used, so no dopant penetrates through the oxide onthese sidewalls because of the large angle away from beingperpendicular.

In FIG. 12 the trench is etched to remove the blocking layer 47 and anyimplanted ions at the bottom of the trenches to a depth H showngenerally at 53.

In FIG. 13 a diffusion step is performed to create P-type doped regions55 and 57. The trenches 45 are filled with an insulator such as silicondioxide in FIG. 14. The trenches can have many different shapes such asthe square shape 100 of FIG. 15 a, the elongated shape 101 of FIG. 15 b,or the dog bone shape 103 of FIG. 15 c. No dopant is introduced on thewalls at the long sides of the structure for any of the geometries.

The FIG. 15 show the location of the implanted dopant 36 and 38following the first and second implants as shown in FIGS. 9 and 10.

After dopant implantation and diffusion to form the doped columns, thetrenches are filled. Typically a dielectric will be used, though it ispossible to fill it with polysilicon and re-crystallize the polysilicon,or to fill the trench with single crystal silicon using epitaxialdeposition. Once the surface is planarized, the active region thatincludes the body, gate dielectric and conductor, and the source regionsshould be placed anywhere there is no trench present to provide channelregions for carrier flow. For the array 104 of FIG. 16, active regionscan be anywhere in the rows and columns between the trenches. Dependingon the dimensions of the trench, polygonal, cellular or stripegeometries are all feasible. A striped geometry might run parallel tothe long axis of the trenches (top row of FIG. 16). A cellular geometrymight enclose each trench as shown on the bottom row 16 b of FIG. 16. Ifa cell is formed at each end of the trench (middle row of FIG. 16), thesource injects carriers around 3 sides, but not at the fourth side. Thecross section for either cellular version is the same through the dopedcolumn and is shown in FIG. 17.

The Use of Trenches Having Different Orientations in Combination withImplants with Dopants Having Different Conductivity Types is illustratedin FIG. 19.

The creation of the active region includes the steps of implanting the Ptype source body region 5 on top of the P columns 36 and 38. A source 7of N type dopant is then implanted on top of the source body regions 5.A gate oxide 6 is deposited and the gate electrode 18 is formed in thegate oxide between the rows 108 and 148 over the sources 7. Finally, thesource electrode is connected to the source and source body region ofeach device.

A variation of the technique that was previously discussed uses theimplantation of dopants of both conductivity types in the active regionof the device. In this variation, the second dopant type is implanted atan angle of 90° and 270° to the first dopant implant, as shown in FIG.19. It provides the needed amount of dopant compensation and/or chargebalance to obtain a high breakdown voltage. Where the structures 11 haveN-type dopants implanted at regions 136 and 138 and P-type dopantsimplanted at regions 36 and 38. A second set of rectangular trenches 35that are perpendicular to the first set of trenches 35 provide thiscapability are shown in FIG. 19. While geometries that allow the dopingof the walls of a single trench with dopants of both conductivity typesis shown in FIG. 23. Unwanted doping of the top region of any sidewallwhich could occur when two dopants are implanted at 90° to each othercan be prevented by using a blocking layer having a greater thicknessalong the top part of the sidewall than previously shown.

A Compatible Termination Structure

A formation of a termination at the device perimeter that is compatiblewith the sequence used in the fabrication of the super-junctionstructure at the center of the device is often a challenge.

In the present embodiment, however, it is possible to form a compatibletermination structure by either using the same process sequence, or byadding one more implants to the existing process sequence. These twopossibilities are discussed in greater detail below.

A Compatible Termination Structure that Requires no Additional ProcessSteps

This termination structure is best understood by referring to FIGS. 21and 22. FIG. 20 shows a top view and FIGS. 21 and 22 show a side view oftrenches 35, 121 and 122 at the termination having different lengths,device 207, dotted line trenched 201 and device 209 and/or having bothdifferent lengths and widths trenches 207, 211 and 200—and differentwidth trenches 207, 201 and 205. The trench length directly determinesthe depth along the sidewall that is implanted on the two walls at theends of each trench while the trench width directly affects the totalcharge introduced in these two sidewalls. By varying the trench lengthand width, both the depth of the junctions formed by the introduceddopant and the total dopant amount that is introduced can be optimized.By also controlling the number and the locations of the trenches thatare etched in the termination region, as shown in FIG. 20, the positionsas well as the depths of the diffused P-type junctions in thetermination region can be optimized to produce the highest breakdownvoltage.

It is also possible to etch trenches that are not generally rectangularin shape (such as crosses 214, squares 215 or circles 216 of FIGS. 24and 25) that may also have different dimensions to etch trenches thatare generally rectangular in shape, but with their axes along a linethat is different from that of the trenches etched in the active regionof the device. Examples of these trenches are shown in FIG. 26.

A Compatible Termination Structure that Requires an Additional ImplantStep

The termination structure uses a second implant step with a dopanthaving the same conductivity type as that of the region containing thetrenches. This additional implant provides dopant that can eitherpartially compensate the dopant from the first implant, or providecharge to balance the dopant introduced by the first implant. By etchinga second set of trenches 123 that are generally rectangular shaped, andthat have their major axis at an angle offset to the axis of the firstset of trenches and by varying the dimensions of the trenches asdiscussed above, it is possible to control both the location and theamount of dopant introduced. Examples of possible termination trenchesof this type are shown in FIG. 26. It is also possible to etch trenchesthat are not generally rectangular in shape (such as squares ortrenches) that may also have different dimensions or to etch trenchesthat are generally rectangular in shape with their axes along a linethat is different from that of the trenches etched in the active regionof the device as is shown in FIGS. 19 and 27. Implanting the firstdopant type along one set of axes and the second dopant type alonganother set of axes that is 90° to the first set of axes provides theneeded amount of dopant compensation and/or charge balance to obtain ahigh breakdown voltage.

I claim:
 1. A semiconductor device formed by a method comprising:providing a semiconductor substrate having first and second mainsurfaces opposite to each other, the semiconductor substrate having aheavily doped region of a first conductivity type at the second mainsurface and having a lightly doped region of the first conductivity typeat the first main surface side; providing in the semiconductor substratea plurality of trenches, with each trench having a first extendingportion extending from the first main surface towards the heavily dopedregion, each trench having a first sidewall surface and a secondsidewall surface, at least one of the plurality of trenches beingcontrolled to extend to a first predetermined depth position differentfrom a second depth position of the other of the plurality of trenches,the at least one of the plurality of trenches also having a first lengthand a first width at least one of which is different from acorresponding second length or second width of the other of theplurality of trenches; forming a blocking layer on the first and secondsidewalls and the bottom of each trench; ion implanting the firstsidewall surface of each trench with a dopant of a second conductivitytype to form a first implanted region of the second conductivity type atthe first sidewall surface, the first implanted region extending atleast partially between the first and second main surfaces of thesemiconductor substrate and not beyond the bottom surface of therespective trench; ion implanting the second sidewall surface of eachtrench with a dopant of the second conductivity type to form a secondimplanted region of the second conductivity type at the second sidewallsurface, the second implanted region extending at least partiallybetween the first and second main surfaces of the semiconductorsubstrate and not beyond the bottom surface of the respective trench;and removing the blocking layer from the first and second sidewalls andbottoms of each trench, wherein in between each set of adjacent ones ofthe plurality of trenches a portion of the lightly doped region of thesemiconductor substrate is disposed laterally between the respectivefirst and second implanted regions of the adjacent trenches.